Disable HTML stack traces by Xdebug

The administrator has installed Xdebug 2.1.1 in our shared PHP 5.3.0 server in order to use its debugger. Now, I can hardly read the stack traces of uncatched exceptions because they are formatted by Xdebug with annoying colours that interact badly with the site's CSS:

Since PHP runs as Apache module, I've tried to disable this feature in an